An antenna with parallel electrically conductive surfaces is described in an article titled “A Metallic Fabry—Perot Directive Antenna” by Nicolas Guerin et al. and published in the IEEE transactions on antennas and propagation Vol 54 No. 1 (2006), pages 220-224. In one surface a two dimensional array of slots is provided. The slots have the effect that an electromagnetic waves pattern between the surfaces leaks out through the slots. This results in a directional antenna pattern. The term “slots” is used here because it is conventional. As used herein, the term “slots” refers to openings of any shape through a surface.
Among others, the main lobe of the directional antenna depends on the two-dimensional wave pattern between the surfaces. This pattern is determined by the feed structure. Due to reciprocity a feed structure may be used for transmission and reception. As used herein, the term feed structure refers to either one or both of transmission and reception, although its operation will be described in terms of transmission.
The article by Guerin et al uses a patch antenna as a feed structure, with a square patch of electrically conductive material located between the surfaces, near the second surface and at the centre of the two dimensional array. Unfortunately, this feed structure allows only for a limited number of antenna patterns. Furthermore, positioning of the patch complicates manufacture antenna. Guerein et al describe that foam may be injected between the surfaces, but it would be desirable to use surfaces on mutually opposite sides of a dielectric sheet. In this case multiple sheets would be needed to realize a patch between the surfaces. Moreover, to realize sharp lobes it is desirable to use a feed structure that produces narrowly directed waves between the surfaces. Also, to realize lobes in different directions it is desirable to use a feed structure that is capable of producing a plurality of narrowly directed waves between the surfaces.